Re: [PATCH v3 8/9] iommu/arm-smmu-v3: Add support for PCI ATS

Robin Murphy
2019-07-01T17:42:01Z

Hi Jean-Philippe,

I realise it's a bit late for a "review", but digging up the original
patch seemed as good a place as any to raise this...

On 17/04/2019 19:24, Jean-Philippe Brucker wrote:
[...]
> @@ -1740,6 +1906,9 @@ static void arm_smmu_detach_dev(struct arm_smmu_master *master)
>
> master->domain = NULL;
> arm_smmu_install_ste_for_dev(master);
> +
> + /* Disabling ATS invalidates all ATC entries */
> + arm_smmu_disable_ats(master);
> }

Is that actually true? I had initially overlooked this entirely while
diagnosing something else and thought that we were missing any ATC
invalidation on detach at all, but even having looked again I'm not
entirely convinced it's bulletproof.

Firstly, the ATS spec only seems to say that *enabling* the ATS
capability invalidates all ATC entries, although I think any corner
cases that that alone opens up should be at best theoretical. More
importantly though, pci_disable_ats() might not actually touch the
capability - given that, it seems possible to move a VF to a new domain,
and if it's not reset, end up preserving now-bogus ATC entries despite
the old domain being torn down and freed. Do we need an explicit ATC
invalidation here to be 100% safe, or is there something else I'm missing?

Robin.